Polar potassium rare earth nitrates K2[RE(NO3)5(H2O)2] (RE = La, Ce, Pr and Nd). I. Crystal growth and crystal structures

Abstract

Using a standard evaporation technique at 311 K, large single crystals of the polar orthorhombic potassium rare earth nitrates K2RE(NO3)5.2H2O, where RE = La, Ce, Pr or Nd, of optical quality and dimensions up to 5 × 5 × 4 cm, were grown from aqueous solutions containing a stoichiometric ratio of potassium and rare earth ions (K:RE = 2:1) and a surplus of nitric acid. Detailed structural and crystal chemical analyses of all four isomorphic compounds based on single-crystal X-ray diffraction data were carried out [space groupFdd2;Z= 8; La compound:a= 11.2814 (6),b= 21.480 (1),c= 12.2589 (4) Å,R= 0.94%; Ce compound:a= 11.263 (3),b= 21.404 (3),c= 12.230 (4) Å,R= 1.92%; Pr compound:a= 11.213 (2),b= 21.392 (4),c= 12.204 (2) Å,R= 1.58%; Nd compound:a= 11.197 (1),b= 21.378 (1),c= 12.195 (1) Å,R= 1.55%]. The main structural feature is the diaquapentanitratolanthanide(III) group, [RE(NO3)5(H2O)2]2−, as stated earlier by Eriksson, Larsson, Niinistö & Valkonen [Acta Chem. Scand. Ser. A, (1980),34, 567–572] for the La compound. The rare earth atoms are surrounded by 12 O atoms, two of which belong to water molecules, the remaining ten belonging to five bidentate nitrate groups. The coordination polyhedron [REO12] is a distorted icosahedron; its geometry is discussed using a simple hard-sphere model.